The Organic Light Emitting Display (OLED) possesses many outstanding properties of self-illumination, low driving voltage, high luminescence efficiency, short response time, high clarity and contrast, near 180° view angle, wide range of working temperature, applicability of flexible display and large scale full color display. The OLED is considered as the most potential display device.
The OLED can be categorized into two major types according to the driving methods, which are the Passive Matrix OLED (PMOLED) and the Active Matrix OLED (AMOLED), i.e. two types of the direct addressing and the Thin Film Transistor (TFT) matrix addressing. The AMOLED comprises pixels arranged in array and belongs to active display type, which has high lighting efficiency and is generally utilized for the large scale display devices of high resolution.
The AMOLED is a current driving element. When the electrical current flows through the organic light emitting diode, the organic light emitting diode emits light, and the brightness is determined according to the current flowing through the organic light emitting diode itself. Most of the present Integrated Circuits (IC) only transmit voltage signals. Therefore, the AMOLED pixel driving circuit needs to accomplish the task of converting the voltage signals into the current signals. The traditional AMOLED pixel driving circuit generally is 2T1C, which is a structure comprising two thin film transistors and one capacitor to convert the voltage into the current.
As shown in FIG. 1, which is a 2T1C pixel driving circuit employed for AMOLED, comprising a first thin film transistor T10, a second thin film transistor T20 and a capacitor C10. The first thin film transistor T10 is a switch thin film transistor, and the second thin film transistor T20 is a drive thin film transistor, and the capacitor C10 is a storage capacitor. Specifically, a gate of the first thin film transistor T10 receives a scan signal Scan, and a source receives a data signal Data, and a drain is electrically coupled to the gate of the second thin film transistor T20 and one end of the capacitor C10; a source of the second thin film transistor T20 is electrically coupled to a power source positive voltage VDD, and a drain is electrically coupled to an anode of the organic light emitting diode D10; a cathode of the organic light emitting diode D10 is electrically coupled to a power source negative voltage VSS; the one end of the capacitor C10 is electrically coupled to the drain of the first thin film transistor T10, and the other end is electrically coupled to the source of the second thin film transistor T20. As the AMOLED displays, the scan signal Scan controls the first thin film transistor T10 to be activated, and the data signal Data enters the gate of the second thin film transistor T20 and the capacitor C10 via the first thin film transistor T10. Then, the first thin film transistor T10 is deactivated. With the storage function of the capacitor C10, the gate voltage of the second thin film transistor T20 can remain to hold the data signal voltage to make the second thin film transistor T20 to be in the conducted state to drive the current to enter the organic light emitting diode D10 via the second thin film transistor T20 and to drive the organic light emitting diode D10 to emit light.
The structure of the pixel driving circuit shown in FIG. 1 is simpler and does not have the compensation function, and thus many defects exist. The more obvious one is: the driving thin film transistor, i.e. the second thin film transistor T20 is long term operating in the positive bias state, which leads to the threshold voltage shift and influences the display quality of the OLED display device.